MICROSCOPIC ILLUSTRATIONS. 2/3 



tensity of the light will be beyond the focus, and the dif- 

 fusion will take place within it, as represented in figure 4. 

 The indications of over correction will be more particularly 

 dwelt upon hereafter. On cutting off the aperture of the 

 nine-tenths plano-convex lens to three-fortieths of an inch 

 the spherical aberration becomes insensible, unless the 

 light of the sun is used, and the distinctness as perfect as 

 the want of light will permit. 



We may now obtain from it the characteristics of an 

 object-glass of very small aperture, free from spherical 

 error, which are as follow : There is the same strength 

 and intensity of light within and without the focus evinced 

 both by its action on the dial-plate and on the artificial 

 star. There is no diffusion of light either within or with- 

 out the radiant point ; the objects are perfectly well-defined 

 without coma or nebulosity (see figures 6, 6', 6 /x ) when in 

 focus, and vanish, exhibiting alike a penumbra in the case 

 of the dial-plate (figure 8 X// ), and a circular disc (figure 9) 

 in that of the artificial star when put out of focus either 

 way. 



CHROMATIC ABERRATION 



Can never be seen in perfection until it is fairly sepa- 

 rated from spherical. It is amusing to see how the two 

 kinds of aberration mutually keep each other in counte- 

 nance. 



The same nine-tenths plano-convex lens, already described 

 with the same aperture, is used in the following observations 

 on the aberration of colour or refrangibility ; the object is 

 a rather large globule of quicksilver, and the illumination 

 that of a southern window on a bright clay (figure 6.) 



